Understanding Variables in Java Programming

1. CSCI 1100/1202 January 16, 2002

2. Why do we need variables? • To store intermediate results in a long computation. • To store a value that is used more than once. • To simplify steps in a piece of code. • To help prevent code duplication. • See Variables.java

3. Variables • A variable is a name for a location in memory • Can be used to store a value for later use. • Can hold a value of a specific type. • Can be modified at any time in the program. • Must be declared before using. • Uses a unique identifier for a name.

4. data type variable name Variable Declaration • A variable must be declared, specifying the variable's name and the type of information that will be held in it int total; int count, temp, result; Multiple variables can be created in one declaration, but it is generally a better practice to put one variable per line with an appropriate comment

5. Value of a variable • A variable can be given an initial value in the declaration int sum = 0; int base = 32, max = 149; • Once assigned a value, the variable name can be used in any expression. • When evaluated, the current value of the variable is substituted for that variable name. • See PianoKeys.java (page 60)

6. Assignment • An assignment statement changes the value of a variable • The assignment operator is the = sign total = 55; • The expression on the right is evaluated and the result is stored in the variable on the left • The value that was in total is overwritten • You can only assign a value to a variable that is consistent with the variable's declared type • See Geometry.java (page 62)

7. Constants • A constant is an identifier that is similar to a variable except that it holds one value for its entire existence • The compiler will issue an error if you try to change a constant • In Java, we use the final modifier to declare a constant final int MIN_HEIGHT = 72;

8. Constants • Give names to otherwise unclear literal values • Facilitate changes to the code • Prevent inadvertent errors • See Variables2.java

9. Primitive Data • There are exactly eight primitive data types in Java • Four of them represent integers: • byte, short, int, long • Two of them represent floating point numbers: • float, double • One of them represents characters: • char • And one of them represents boolean values: • boolean

10. Type byte short int long float double Storage 8 bits 16 bits 32 bits 64 bits 32 bits 64 bits Min Value -128 -32,768 -2,147,483,648 < -9 x 1018 +/- 3.4 x 1038 with 7 significant digits +/- 1.7 x 10308 with 15 significant digits Max Value 127 32,767 2,147,483,647 > 9 x 1018 Numeric Primitive Data • The difference between the various numeric primitive types is their size, and therefore the values they can store:

11. Characters • Achar variable stores a single character from the Unicode character set • A character set is an ordered list of characters, and each character corresponds to a unique number • The Unicode character set uses sixteen bits per character, allowing for 65,536 unique characters • It is an international character set, containing symbols and characters from many world languages • Character literals are delimited by single quotes: 'a' 'X' '7' '$' ',' '\n'

12. uppercase letters lowercase letters punctuation digits special symbols control characters A, B, C, … a, b, c, … period, semi-colon, … 0, 1, 2, … &, |, \, … carriage return, tab, ... Characters • The ASCII character set is older and smaller than Unicode, but is still quite popular • The ASCII characters are a subset of the Unicode character set, including:

13. Boolean • Abooleanvalue represents a true or false condition • A boolean can also be used to represent any two states, such as a light bulb being on or off • The reserved wordstrueandfalseare the only valid values for a boolean type boolean done = false;